A device for improving visual ability of a human is disclosed in JP-A-2000-203335. The device in the document outputs a light with visible wavelength and an ultraviolet ray toward the frontward of a vehicle. By virtue of the ultraviolet ray, clothes of a passenger become distinctly visible to the driver.
In addition, devices for improving visual ability of a human by utilizing stochastic resonance are described in Japanese patent applications No. 2004-364997 and No. 2004-366489.
The stochastic resonance (SR) in a human brain is discussed in “Functional stochastic resonance in the human brain: Noise induced sensitization of baroreflex system”, Hidaka, et al., proceedings of Bionics and Physiology Engineering Symposium, Vol. 15, p. 261-264.
The stochastic resonance in a living body is a phenomenon in which an input of proper noise to a sensory nerve cell enhances its sensitivity to a sub-threshold input signal. It is experimentally shown that the stochastic resonance in a living body enhances vital functions of a human such as perception, regulation and action.
Hereafter, a stochastic resonance is described with reference to FIGS. 10A-10C. FIG. 10A shows a schematic diagram of a sensory nerve cell as a nonlinear system. FIG. 10B shows threshold type input/output characteristics of the system. FIG. 10C shows an output S/N of the system relative to input noise intensity.
As seen in FIG. 10B, the system generally does not respond to an input signal with intensity below a threshold because it has threshold type input/output characteristics. However, if wideband noise is inputted to the system, it responds to some sub-threshold input signals.
In addition, as shown in FIG. 10C, if the wideband noise is too faint or too intensive compared to a threshold irrespective of the intensity of the input signal, the noise decreases the signal-to-noise ratio (S/N) of output signals from the system. With moderate noise intensity the signal-to-noise ratio becomes larger and with the optimum intensity it becomes at its maximum.